Replace string GNU Triples with llvm::Triple in MCAsmBackend subclasses and...

Replace string GNU Triples with llvm::Triple in MCAsmBackend subclasses and create*AsmBackend(). NFC.

Summary:
This continues the patch series to eliminate StringRef forms of GNU triples
from the internals of LLVM that began in r239036.

Reviewers: echristo, rafael

Reviewed By: rafael

Subscribers: rafael, llvm-commits, rengolin

Differential Revision: http://reviews.llvm.org/D10243

llvm-svn: 239464